This invention relates to a method of growing silicate glass layers, such as phospho-silicate glass layers (hereinafter referred to as PSG layers), on semiconductor substrates employing a chemical vapour deposition process. More particularly, this invention relates to an improvement of the method to enable the impurity concentration doped in the silicate glass to be made uniform for each of the semiconductor substrates processed in one batch at the same time.
It is widely known that a sealed tube made of quartz or stainless steel is usually employed for growing silicate glass layers, such as PSG layers, on semiconductor substrates employing the chemical vapour deposition process. The sealed tube is provided with an electric heater which surrounds the sealed tube to heat the semiconductor substrates placed therein, one inlet at one end of the sealed tube, a second inlet at the same one end of the sealed tube or along the semiconductor substrates placed therein, one outlet at the other end of the sealed tube, and a vacuum pump which causes reactive gases to flow inside the sealed tube at a specific low pressure. When the chemical vapour deposition process is put into operation, the vacuum pump is operated to cause the reactive gases containing silane (SiH.sub.4), phosphine (PH.sub.3) and oxygen (O.sub.2) to flow into the sealed tube through the inlets and to flow out of the sealed tube through the outlet. The length of such a sealed tube is usually selected to be 2 meters and the intermediary portion, with a length of approximately 1 meter, of the sealed tube is usually kept at a uniform temperature by the heater. A plurality of semiconductor wafers are arranged on a wafer holder or boat placed in the uniform temperature zone of the sealed tube in the vertical or inclined position and in the transverse direction with respect to the sealed tube. The reactive gases containing silane (SiH.sub.4), phosphine (PH.sub.3) and oxygen (O.sub.2) flowing inside the sealed tube make chemical reaction in the neighborhood of the semiconductor wafers to deposite PSG layers on each of the semiconductor wafers.
When growing PSG layers on each of the semiconductor wafers standing side by side in such a long sealed tube reactor, it is fairly difficult to make the conditions of chemical vapour deposition process uniform at each location along the long tube. This of course causes an adverse effect for the uniform impurity concentration doped in PSG grown by the process. It is extremely important to make the impurity or phosphorus concentration doped in the PSG uniform for each of the wafers processed in one batch.
In reality, however, no effective methods were available in the prior art for making the impurity or phosphorus concentration doped in PSG uniform for each of the semiconductor wafers aligned along the flow of the reactive gasses but in the transverse direction to the flow of the reactive gases. Therefore, a practical but inefficient method was employed to make the impurity or phosphorus concentration doped in PSG uniform for all the wafers processed in one batch at the same time. Namely, conditions optimum for making the impurity or phosphorus concentration uniform for all the wafers were determined, on a trial and error basis, by repetition of trials made with varied parameters, such as the temperature, the quantity of gas supplied for the reaction, the internal pressure of the sealed tube, the quantity of substrates processed in one batch, and the shape and length of the sealed tube. It is clear that this trial-and-error-type method is inevitably involved with inefficiency, incorrectness, longer time consumption and inconvenience.